Class Details by Steve Llanso of Sweat House Media

Data from KO&G 8 - 1923 Locomotive Diagrams book supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange and from Data from Record of Recent Construction #91 (Baldwin Locomotive Works, 1918), p.20-21.A summary of the Muskogee Company empire appears in the Muskogee Company records, DeGolyer Library, Southern Methodist University, [], last accessed 26 April 2015; and "Missouri Kansas & Gulf 300-Class 2-6-6-2s", Muskogee Roads Historical &Technical Society's Web Site, last accessed at https://condrenrails.com/FSVB/Midland%20Valley/300-class-2-6-6-2s.htm on 10 June 2018, (Thanks to Chris Hohl for his 2 May 2018 email correcting the boiler pressure.) Works numbers were 38574-38578 in November 1912. . Works numbers were 38574-38578 in December 1912.

See Locobase 17 for a Handbook of Texas description of the KO&G, which succeeded the MO&G after its bankruptcy in 1919. In its palmier days (e.g., 1912), the Gulf Route treated itself to these light-footed Mallets. As the Baldwin record stated, Mallet locomotives, "...under favorable conditions, are specially suitable for heavy service onlines where wheel loads are limited on account of comparatively light tracks and bridges." And on the MO&G, rail weights could be as low as 70 lb/yard (35 kg/metre).
The locomotives had big boilers for their size and 13" (330 mm) piston valves on all four cylinders to aid steam distribution. They were rated to pull 3,000 short tons (2,727 metric tonnes) up a 0.6% grade when the train's frictional resistance did not exceed 6 lb/ton (3 kg/metric tonne).
The caption under a Rick Morgan photograph states "The entire class was replaced by the 500-class 2-10-2s in 1927." What isn't clear is whether the relatively young class took up new duties, was sold to some other railroads, or scrapped.